The most logical answer is that Buddha was a Negro or had Negroid features, which some Asian populations still have, such as the Aeta and Agta of the Philippines and the Semang of Malaysia.
Semang man from 1906 (wikipedia)
The image probably harks back to an era before the Negroid population of Asia reduced and in fact there is no reason to believe that Siddartha Gautama couldn’t have had such features as some Tibetans and Nepalese retained that kind of hair texture into the 20th century and some of them may still do.
You should also note that many Buddha paintings feature him with a black complexion and it is only reasonable that he should retain the hair texture currently associated with Africans.
Coupled with the widow's peak in most of his sculptures and paintings the Buddha image probably derive’s from an earlier population on mainland Indian who may have looked like the Jarawa of the Andaman Islands, as can be seen in these Jarawa people.
Note the Jarawa woman’s widow’s peak and the sideburns of the Jarawa man. The man also has the widow’s peak which is visible when observed closely although he is in profile.
You should also note that the folds in Buddha's neck are commonly found in Africans and South Indians and the earlobes stretched by rings are still found in contemporary Africans.